Constructing a Scene
Since a scene is a dramatic unit of a larger story, each scene should be built in the same way as your screenplay. Once you have the scene fleshed out, go back and edit it. Is there anything from the beginning that you can trim? Try and make the scene as tight as possible; begin the scene as closely to the end as you can. You probably have a good idea about how you want it to end to increase either the drama or the comedic effect; but you don't want to tax your audience's patience by including anything superfluous in the beginning of the scene. If a scene's length runs more than two pages, go back and see if anything can be edited out. Every scene has its own flow and rhythm. If it feels like it's the right length-- long or short--that is something as a writer that you need to figure out.